The present invention relates to a mobile data communication system, and in particular to a technique for maintaining a connection while a mobile terminal is moving between networks.
As small-sized and light weight mobile terminals, such as notebook computers and PDAs (Personal Digital Assistants), spreads and the Internet spreads explosively, an environment that allows use of a mobile terminal even in a visiting place besides home or an office is established, and mail and Internet access utilizing a mobile terminal is becoming popular.
Furthermore, VoIP (Voice over IP) and ALL IP networks are drawing the attention. In the VoIP, data obtained by digitizing analog sound information are packetized and transmitted over an IP network to implement speech communication. In the ALL IP networks, all data are carried on IP frames to conduct communication in mobile communication networks.
In general, an IP network is formed by mutually connecting a plurality of networks (called subnetworks) differing in network number. Each subnetwork has a unique IP address group assigned thereto. Therefore, each of mobile terminals connected to a subnetwork is provided with an IP address included in the unique IP address group assigned to the subnetwork. As for packets transferred between subnetworks, the packets are forwarded on the basis of network numbers. Whenever the mobile terminal moves to a different subnetwork, therefore, the mobile terminal needs to be assigned a different IP address.
If a mobile terminal using IPv4, which is widely used as a communication protocol of the Internet at the present time, moves between subnetworks, then packets directed to an IP address that has been utilized in a network before movement are delivered to the subnetwork of the network number before the movement. In a subnetwork having a different network number after the movement, therefore, it is impossible to maintain the connection by using the IP address utilized in the subnetwork before the movement as it is.
As a communication protocol of the Internet, IPv6 having an address space expanded to 128 bits is spreading. As for IPv6, a technique called Mobile IPv6 (draft-ietf-mobileip-ipv6-13.txt) has been proposed. In order to solve the problem, Mobile IPv6 makes it possible to keep the connection in succession even if a mobile terminal has moved to a different subnetwork. Standardization thereof is under examination in the Internet Engineering Task Force.
In the subnetworks, a subnetwork to which a mobile terminal usually belongs (called home network) is defined According to IPv6. The mobile terminal is assigned a home address as an address to be used in the home network. In the subnetwork, there is provided a Home Agent device (referred to as HA device) having a function of managing location information of mobile terminals for which the subnetwork has been defined as the home network.
Each subnetwork is assigned a high-order section of IPv6 addresses as a prefix address. At least one router called advertising router is provided in each subnetwork. The advertising router advertises a Router Advertisement including a prefix address of the subnetwork to mobile terminals in the subnetwork via a base station. Thereby, the advertising router notifies mobile terminals of a prefix address of a subnetwork to which each mobile terminal has moved or belongs. Furthermore, also in the case where a mobile terminal in a subnetwork has requested a notice of information of the subnetwork to which the mobile terminal belongs at the present time, the advertising router responds by the Router Advertisement.
In the subnetwork to which each mobile terminal has moved, the mobile terminal obtains the prefix address of the subnetwork in which the mobile terminal is present, from the Router Advertisement. By using the address auto configuration function or DHCP (Dynamic Host Configuration Protocol) of IPv6, each mobile terminal gets an address assigned. As a result, each mobile terminal generates a care-of address to be temporarily used in the subnetwork to which the mobile terminal has moved.
The mobile terminal notifies the HA device of the generated care-of address. Thereafter, the HA device captures an IPv6 packet that has arrived at the home address of each mobile terminal, encapsulates the IPv6 packet, and sends it to the care-of address. The mobile terminal decapsulates the encapsulated packet that has arrived at the care-of address, and receives the IPv6 packet that has arrived at the home address.
In this way, the HA device transfers a packet that has arrived at a home address of a home network of a mobile terminal to a care-of address by using the Mobile IPv6 technique. Thereby, the mobile terminal can continuously receive packets destined to the home address. As a result, the mobile terminal can move while maintaining the connection with the opposite party of communication.
In the case where a mobile terminal has moved to a different subnetwork, the mobile terminal cannot detect the fact that it has moved to the different subnetwork until it receives a prefix address of subnetwork information transmitted from the advertising router in the subnetwork to which the mobile terminal has moved. In addition, until a care-of address is generated and registered in the HA device, packets transmitted to the mobile terminal via the HA device are not sent to a new care-of address.
If movement between subnetworks frequently occurs when data are actually flowing such as in the case where a mobile terminal is moving fast or the case where handoff between base stations frequently occurs in a mobile network, packet loss occurs a lot of times. If packet loss occurs a lot of times, the throughput lowers and the communication quality degradation such as sound intermission in the VoIP.
As a result, a further improved mobile data communication technique is demanded.